FIG. 1 depicts a schematic view of a conventional liquid crystal display device 11 using an edge-light-type surface light source device. This liquid crystal display device 11 includes a surface light source device 12 and a liquid crystal panel 15.
In the surface light source device 12, a point source of light 18 using an LED is placed so as to face an end face (a light incident end face) of a light guide plate 17 molded of transparent resin, a diffusion plate 13 and two prism sheets 14 are stacked on an upper surface (or a light emission surface) of the light guide plate 17, and a reflector plate 16 faces a lower surface of the light guide plate 17. Note that the point source of light 18 is implemented in a substrate 20. A liquid crystal panel 15 is placed on the prism sheets 14 via a rim sheet 19 (or black frame).
Thus, light emitted from the point source of light 18 enters the inside of the light guide plate 17 from an end face of the light guide plate 17, propagates through the inside of the light guide plate 17, and is emitted from an approximately entire upper surface of the light guide plate 17. Light emitted from the upper surface of the light guide plate 17 passes through the diffusion plate 13 and the prism sheets 14 to illuminate the liquid crystal panel 15 from a back surface side. Also, light leaking from the lower surface of the light guide plate 17 is reflected off the reflector plate 16 and returns to the inside of the light guide plate 17 for reuse of light.
In the surface light source device 12 as described above, in addition to uniform luminance, high luminance, and low cost, a large light emission area (a small area other than a light emission region) and a thin thickness are demanded. In particular, when the surface light source device 12 is incorporated in a portable device, a demand for reducing the thickness of the surface light source device 12 is increasing more according to reduction in thickness of a portable device
The size of each component of a surface light source device used in general is as follows:                the sum of the thicknesses of the substrate and the point source of light: 600 μm;        the height of the light emission window of the point source of light: 300 μm;        the thickness of the prism sheet: 62 μm (per sheet);        the thickness of the light diffusion plate: 55 μm;        the thickness of the light guide plate: 300 to 650 μm;        the thickness of the reflector plate: 60 μm; and        the thickness of the rim sheet: 55 μm        
Thus, this surface light source device has a thickness on the order of 600 μm on a point source of light side and approximately 539 to 889 μm on a light guide plate side even if the thickness of the rim sheet is excluded. Therefore, it is desired to thin the thickness on a light guide plate side occupying most of the area of the surface light source device.
What occupies most of the thickness of the surface light source device is the light guide plate. However, when the thickness of the light guide plate is thinner than the height of the light emission window of the point source of light, an amount of light emitted from the point source of light but not entering the light guide plate is increased, thereby decreasing light use efficiency of the surface light source device. Therefore, the thickness of the light guide plate is restricted by the height of the light emission window of the point source of light, and it is difficult to make the thickness of the surface light source device thinner than the height of the light emission window of the point source of light. Similarly, when the light source is a cold-cathode tube, it is difficult to make the thickness of the light guide plate thinner than the diameter of the cold-cathode tube.
(Regarding Patent Document 1)
FIG. 2 is a side view of a liquid crystal display device 21 disclosed in Japanese Unexamined Patent Application Publication No. H5-53111 (Patent Document 1). In a surface light source device 22 for use in this liquid crystal display device 21, in order to allow light from a fluorescent tube 23 to enter a light guide plate having a thickness thinner than the fluorescent tube 23, a tapered portion 25 is provided at a portion of the light guide plate having a thin thickness, that is, at an end of a light guide plate body 24. An end face of the tapered portion 25 has a height approximately equal to the diameter of the fluorescent tube 23, and the fluorescent tube 23 faces the end face. And, light entering from the end face of the tapered portion 25 is guided to the light guide plate body 24 by being totally reflected off the front and back surfaces of the tapered portion 25, and is emitted from an upper surface of the light guide plate body 24 to a liquid crystal panel 26.
The surface light source device 22 disclosed in Patent Document 1 has an object of guiding light of the fluorescent tube 23 to the light guide plate without leakage. To this end, the height of the end face of the tapered portion 25 is set approximately equal to the diameter of the florescent tube 23 to allow light of the fluorescent tube 23 to be guided to the tapered portion 25 without leakage. However, in the source light source device 22, light leakage cannot be prevented in the tapered portion 25. Therefore, light leaking from the tapered portion 25 is viewed as glowing from an observer side, and an edge of a display unit (screen) of the liquid crystal display device emits light with high luminance to degrade the quality of the display unit.
The reason why light leakage from the tapered portion 25 cannot be prevented with this structure of the surface light source device 22 is described by using FIG. 3. Now consider light that is prone to leak most from the tapered portion 25. If leakage of this light that is prone to leak most is prevented, it can be said that light leakage from the tapered portion 25 is eliminated in the surface light source device 22. Light that is prone to leak most is a light beam L having the largest light guide angle (an angle formed with a horizontal plane) among light going out from the fluorescent tube 23 and entering the tapered portion 25. Therefore, consider the structure in which the light beam L having the largest light guide angle does not leak from the tapered portion 25 and the thickness of the light guide plate body 24 can be made as thin as possible. To find this structure, as depicted in FIG. 3, consider the condition for allowing the light beam L having a largest light guide angle α to be totally reflected off an upper end (a point A) of an inclined surface of the tapered portion 25 and then again totally reflected off a point B of a lower surface of the light guide plate to be reflected off an upper surface (a point C) adjacent to the tapered portion 25 of the light guide plate body 24. Note that a short flat-shaped portion is depicted at an end face portion of the tapered portion 25 in FIG. 3 merely for convenience of depiction, and the length of the short flat-shaped portion can be thought as being infinitely short.
First, the largest light guide angle α of light entering the tapered portion 25 is determined bysin α=1/n  (Equation 1)                (where n is a refractive index of the light guide plate).        
An angle of incidence at which the light beam L having this largest light guide angle α enters the point A positioned on the inclined surface having an angle of inclination θ is 90°−θ−α. Therefore, a condition of total reflection of the light beam L off the inclined surface isθ≦90°−2α  (Equation 2).
Also, an angle of incidence at which the light totally reflected off the point A enters a lower surface of the tapered portion 25 is 90°−2·θ−α. Therefore, a condition of total reflection of the light beam L off the point B of the lower surface isθ≦45°−α  (Equation 3).If this Equation 3 is satisfied, the light beam L totally reflected off the point B is also totally reflected off the point C of the light guide plate body 24.
Therefore, it can be found from Equation 2 and Equation 3 that, in order to allow the light beam L to be totally reflected off the point A, the point B, and the point C,θ≦45°−α  (Equation 4)is satisfied. However, if the angle of inclination θ of the tapered portion 25 is small, there is a possibility that light totally reflected off the upper end of the inclined surface of the tapered portion 25 and then totally reflected off the lower surface of the light guide plate again enters the inclined surface of the tapered portion 25 and leaks from the tapered portion 25. Also, if the angle of inclination θ is small, the length of the tapered portion 25 is long, and therefore the angle of inclination θ is desired to be as large as possible as long as Equation 4 is satisfied. Therefore, the angle of inclination θ is set to have a value as large as possible within a limitation of satisfying Equation 4. That is,θ−45°−α  (Equation 5).
Then, when the height of the end face of the tapered portion 25 is taken as T, the length of the tapered portion 25 is taken as X, and the difference in height of the inclined surface of the tapered portion 25 is taken as Y, from FIG. 3, the length X and the difference in height Y of the tapered portion 25 become
                                                        X              =                            ⁢                                                T                  ⁢                                                                          ⁢                                      cot                    ⁡                                          (                                              α                        +                                                  2                          ·                          θ                                                                    )                                                                      +                                                      (                                          T                      -                      Y                                        )                                    ⁢                                      cot                    ⁡                                          (                                              α                        +                                                  2                          ·                          θ                                                                    )                                                                                                                                                              =                                ⁢                                                      (                                                                  2                        ⁢                        T                                            -                      Y                                        )                                    ⁢                                      cot                    ⁡                                          (                                              α                        +                                                  2                          ·                          θ                                                                    )                                                                                  ,                                                                                  and                                                      Y        =                  X          ⁢                                          ⁢          tan          ⁢                                          ⁢                      θ            .                                                          By solving these equations for X and Y and using Equation 5, the following Equation 6 and Equation 7 can be obtained.
                    x        =                              2            ⁢                          a              ⁡                              (                                  1                  +                  a                                )                                      ×            T                                1            +                          2              ⁢              a                        -                          a              2                                                          (                  Equation          ⁢                                          ⁢          6                )                                y        =                              2            ⁢                          a              ⁡                              (                                  1                  -                  a                                )                                      ×            T                                1            +                          2              ⁢              a                        -                          a              2                                                          (                  Equation          ⁢                                          ⁢          7                )            
where, α=tan α=tan (45°−θ)
Also, the light guide plate body 24 has a thickness t, which is represented by the following Equation 8.
                    t        =                              T            -            y                    =                                                    (                                  1                  +                                      a                    2                                                  )                            ×              T                                      1              +                              2                ⁢                a                            -                              a                2                                                                        (                  Equation          ⁢                                          ⁢          8                )            
As a light guide plate material, consider acrylic resin or polycarbonate resin (PC resin), which is a typical light guide plate material, and it is assumed for calculation that the light guide plate has a refractive index n as                n=1.49 (in the case of acrylic resin), and        n=1.59 (in the case of polycarbonate resin).With this, from Equation 1, the largest light guide angle α becomes        α=42.16° (in the case of acrylic resin), and        α=38.97° (in the case of polycarbonate resin).From Equation 3, the angle of inclination θ of the tapered portion 25 becomes        θ=2.84° (in the case of acrylic resin), and        θ=6.03° (in the case of polycarbonate resin).        
Also, in Patent Document 1, since the height of the end face of the tapered portion 25 is described as T=4.10 mm, by using this value of the height T and the value of a above, from Equations 6 to 8, the length X of the tapered portion 25 and the difference in height Y, and the thickness t of the light guide plate body 24 are found as follows. When the light guide plate material is acrylic resin, since T=4.10 mm and α=42.16° (in Equations 6 to 8, α=tan α=0.91),                X=7.10 mm,        Y=0.35 mm, and        t=3.75 mm.        
Similarly, when the light guide plate material is polycarbonate resin, since T=4.10 mm and α=38.97° (in Equations 6 to 8, α=tan α=0.81),                X=6.11 mm,        Y=0.65 mm, and        t=3.45 mm.        FIG. 4 shows the calculation results as above.        
According to FIG. 4, the thickness t of the light guide plate body 24 is 3.75 mm (in the case of acrylic resin) or 3.45 mm (in the case of polycarbonate resin). By contrast, in the liquid crystal display device 21 disclosed in Patent Document 1, the height of the end face of the tapered portion 25 is T=4.10 mm and the thickness of the light guide plate body 24 is t=2.2 mm. The value of t=2.2 mm is considerably thinner than the value of the thickness t found though the calculation above (in FIG. 4). Therefore, light inevitably leaks from the tapered portion 25.
Therefore, in the surface light source device 22 disclosed in Patent Document 1, it is impossible to prevent light leakage from the tapered portion 25. Alternatively, in the surface light source device 22 disclosed in Patent Document 1, at least light leakage from the tapered portion 25 is not considered at all. Alternatively, to eliminate light leakage, the thickness t of the light guide plate body 24 cannot be made thinner with respect to the thickness T of the tapered portion 25, or the length X of the tapered portion 25 has to be made very long.
(Regarding Patent Documents 2 and 3)
Japanese Unexamined Patent Application Publication No. 2004-69751 (Patent Document 2) and Japanese Unexamined Patent Application Publication No. 2005-285389 (Patent Document 3) each also disclose a surface light source device. However, in the surface light source device described in Patent Document 2, light leaking from a light guide plate to the outside cannot be sufficiently suppressed, and leaking light glows on the display surface of a liquid crystal display device to degrade the quality of the liquid crystal display device. Also, in the surface light source device described in Patent Document 3, light is absorbed at a light reflector plate, and light reflected off the light reflector plate leaks from a light incident end surface, thereby decreasing light use efficiency. Note that technical problems of the surface light source devices in Patent Documents 2 and 3 are specifically described in Patent Document 4.
(Regarding Patent Document 4)
Therefore, the applicant of the present invention has disclosed in the previously-submitted international application (PCT/JP2008/60610: Patent Document 4) a surface light source device allowing the thickness of a light guide plate body to be made sufficiently small compared with the height of a light incident end face and also further decreasing light leakage from a tapered light introducing unit. This surface light source device 31 includes a point source of light 32 using an LED and a light guide plate 33 as depicted in FIG. 5. The light guide plate 33 is formed by providing a light introducing unit 35 to an end of a light guide plate body 34, and is molded with transparent resin having a high refractive index. The light introducing unit 35 has a thickness thicker than that of the light guide plate body 34, and has its end face which the point source of light 32 faces. In the light introducing unit 35, a protruding portion in a shape of an approximately half of a frustum of a cone protrudes from a surface on the same side of a light emission surface 39 of the light guide plate body 34 to increase the thickness of the light introducing unit 35. An outer perimeter surface of the protruding portion forms an inclined surface 37. Along this inclined surface 37, a directivity conversion pattern 38 is formed. Note that, although not shown, on a side (back side) of the light guide plate body 34 opposite to the light emission surface 39, many light emitting means 40 (refer to FIG. 6) each in a prism shape are formed for causing light guided in the light guide plate body 34 to be reflected and emitted from the light emission surface 39.
As such, in this surface light source device 31, as depicted in FIG. 6, light L emitted from the point source of light 32 enters the inside of the light introducing unit 35 from the light incident end face 36, and is totally reflected off the directivity conversion pattern 38 and a lower surface of the light introducing unit 35 or passes through the light introducing unit 35 to be guided to the light guide plate body 34 having a thin thickness. The light guided to the light guide plate body 34 is totally reflected or diffused by the light emitting means 40 to be emitted approximately uniformly from the light emission surface 39.
Then, in the above-structured surface light source device 31, for example, when it is assumed that                the refractive index of the light guide plate 33: n=1.59;        the vertical angle of each mountain-shaped portion of the directivity conversion pattern 38: φ3=120°;        the thickness of the end face of the light introducing unit 35: T=0.31 mm;        the thickness of the light guide plate body 34: t=0.18 mm;        the length of an upper surface of the light introducing unit 35: s1=2.50 mm;        the length of the light introducing unit 35: s2=3.19 mm; and        the angle of inclination of the inclined surface 37: θ1=15.3°,light leakage from the light guide plate 33 can be eliminated in a plane perpendicular to the light emission surface 39.        
Also, in a plan view of the light introducing unit 35 depicted in FIG. 7 (a), an angle (hereinafter referred to as a half-width expected angle) formed by a line segment connecting one end f1 of the light emission window 32a of the point source of light 32 and a center g of an edge of the directivity conversion pattern 38 on an inner perimeter side and a line segment connecting a center f0 of the light emission window 32a and the center g of the edge of the directivity conversion pattern 38 on the inner perimeter side is assumed to be 20°. Also, an angle formed by a line segment connecting another end f2 of the light emission window 32a of the point source of light 32 and the center g of the edge of the directivity conversion pattern 38 on the inner perimeter side and a line segment connecting the center f0 of the light emission window 32a and the center g of the edge of the directivity conversion pattern 38 on the inner perimeter side is assumed to be 20°. In this example, since the occupied area of the light introducing unit 35 is increased, a dead space of the light guide plate 33 is increased. However, due to refraction of light entering the light introducing unit 35, a spread of light at the center portion of the directivity conversion pattern 38 in a horizontal direction is decreased to be smaller than 20°. At a portion away from the center of the directivity conversion pattern 38, the spread of light in a horizontal direction is further decreased. With this, light leakage can be prevented in the entire directivity conversion pattern 38. Therefore, leaked light is extremely decreased in a plane parallel to the light emission surface 39, andleaked light/incident light≦2%can be satisfied.
Therefore, in the surface light source device disclosed in Patent Document 4, even when the angle of inclination θ1 of the inclined surface 37 is large, such as 15.3°, it is possible to extremely decrease light leakage from the light guide plate 33.
However, in the market of surface light source devices, together with reduction in thickness, a light guide plate with a small dead space is strongly demanded. Therefore, for commercialization of the surface light source device 31, consideration of decreasing the occupied area of the light introducing unit 35 is required. As a result of decreasing the occupied area of the light introducing unit 35 as such, in the practical surface light source device 31, as depicted in FIG. 7(b), an angle (a half-width expected angle) formed by a line segment connecting one end f1 of the light emission window 32a of the point source of light 32 and the center g of the edge of the directivity conversion pattern 38 on the inner perimeter side and a line segment connecting the center f0 of the light emission window 32a and the center g of the edge of the directivity conversion pattern 38 on the inner perimeter side is 30°. Δs a result, compared with the case of FIG. 7(a) where this angle is 20°, leaked light is increased, that is,leaked light/incident light≦15%.
Therefore, when the occupied area of the light introducing unit 35 is decreased, leaked light reaches as much as 15%, disadvantageously resulting in a decrease in light use efficiency of the surface light source device and bringing a decrease in luminance of the luminescent surface (the light emission surface 39). Also, as depicted in FIG. 8, due to light leaking from the directivity conversion pattern 38, a light emission region J with high luminance occurs at the edge of the light introducing unit 35, thereby degrading the quality of the surface light source device 31.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 1-15-53111    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2004-69751    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2005-285389    [Patent Document 4] PCT/JP2008/60610